Fuel cell cathode with redox couple

ABSTRACT

Fuel cell oxygen electrode and instant startup fuel cells employing such oxygen electrode. The oxygen electrode operates through the mechanism of redox couples which uniquely provide multiple degrees of freedom in selecting the operating voltages available for such fuel cells. Such oxygen electrodes provide the fuel cells in which they are used a “buffer” or “charge” of oxidizer available within the oxygen electrode at all times.

RELATED APPLICATIONS

The present invention is a continuation of co-pending U.S. patentapplication Ser. No. 10/185,414, which is assigned to the same assigneeas the current application, entitled “Fuel Cell Cathode With RedoxCouple”, filed Jun. 28, 2002, which is a continuation-in-part of Ser.No. 09/797,332 now U.S. Pat. No. 6,620,539, which is assigned to thesame assignee as the current application, entitled “Novel Fuel CellCathodes and Their Fuel Cells”, filed Mar. 1, 2001, the disclosures ofwhich are hereby incorporated by reference.

FIELD OF THE INVENTION

The instant invention relates to generally to useful cathode activematerials for fuel cells, more specifically to their use as the cathodematerial for Ovonic instant startup alkaline fuel cells. These inventivecathodes open up a tremendous number of degrees of freedom in fuel celldesign by utilizing reduction/oxidation (redox) couples, such asmetal/oxide couples, or simply couples which provide electrochemicaloxidizer, preferably oxygen, to the fuel cell electrolyte forelectrochemical “combustion”. These redox couples, due to theirelectrochemical potential, provide the fuel cells employing them with anincreased operating voltage that is adjustable by varying the redoxcouple used. Additionally the redox couple provide the fuel cell withthe ability to store oxidizer within the electrode which not onlyprovides for instant startup, but also provides the capability toprovide short surge bursts of energy during demand surges and alsoallows for recapture of regenerative energy.

BACKGROUND OF THE INVENTION

The instant application for the first time provides oxygen electrodes,and fuel cells using such electrodes, which use oxide couples to yield awide selection of operating voltages. Specifically, the instantinventors have determined materials, which used in combination withhydrogen-side electrodes, particularly with those constructed of Ovonic(Trademark of Energy Conversion Devices, Inc.) hydrogen storagematerial, both of which, in combination, yield high performance fuelcells having hydrogen storage capacity within the hydrogen electrode andoxygen electrodes which take advantage of low-cost, in comparison withthe traditional platinum electrodes, oxide couples which allow selectionof specific ranges of operating voltage of the electrochemical cellswith a broad operating temperature range and the opportunity to provideinstant-start by use of the hydrogen storage capability of theshort-range order available in the material of the Ovonic hydrogenelectrode.

As the world's human population expands, greater amounts of energy arenecessary to provide the economic growth all nations desire. Thetraditional sources of energy are the fossil fuels which, when consumed,create significant amounts of carbon dioxide as well as other moreimmediately toxic materials including carbon monoxide, sulfur oxides,and nitrogen oxides. Increasing atmospheric concentrations of carbondioxide are warming the earth; creating the ugly specter of globalclimatic changes. Energy-producing devices which do not contribute tosuch difficulties are needed now.

A fuel cell is an energy-conversion device that directly converts theenergy of a supplied gas into an electric energy. Highly efficient fuelcells employing hydrogen, particularly with their simple combustionproduct of water, would seem an ideal alternative to current typicalpower generations means. Researchers have been actively studying suchdevices to utilize the fuel cell's potential high energy-generationefficiency.

The base unit of the fuel cell is a cell having an oxygen electrode, anhydrogen electrode, and an appropriate electrolyte. Fuel cells have manypotential applications such as supplying power for transportationvehicles, replacing steam turbines and power supply applications of allsorts. Despite their seeming simplicity, many problems have preventedthe widespread usage of fuel cells.

Presently most of the fuel cell R & D is focused on P.E.M. (ProtonExchange Membrane) fuel cells. Unfortunately, the P.E.M. fuel cellsuffers from relatively low conversion efficiency and has many otherdisadvantages. For instance, the membrane and the electrolyte for thesystem is acidic. Thus, noble metal catalysts are the only useful activematerials for the electrodes of the system. Unfortunately, not only arethe noble metals costly, they are also susceptible to poisoning by manygases, specifically carbon monoxide (CO). Also, because of the acidicnature of the P.E.M fuel cell electrolyte, the remainder of thematerials of construction of the fuel cell need to be compatible withsuch an environment, which again adds to the cost thereof. The protonexchange membrane itself is quite expensive, and because of it's lowproton conductivity at temperatures below 80° C., inherently limits thepower performance and operational temperature range of the P.E.M. fuelcell as the PEM is nearly non-functional at low temperatures. Also, themembrane is sensitive to high temperatures, and begins to soften at 120°C. The membrane's conductivity depends on water and dries out at highertemperatures, thus causing cell failure. Therefore, there are manydisadvantages to the P.E.M. fuel cell which make it somewhat undesirablefor commercial/consumer use.

The conventional alkaline fuel cell has some advantages over P.E.M.fuels cells in that they have higher operating efficiencies, they useless costly materials of construction, and they have no need forexpensive membranes. The alkaline fuel cell also has relatively higherionic conductivity in the electrolyte, therefore it has a much higherpower capability. While the conventional alkaline fuel cell is lesssensitive to temperature than the PEM fuel cell, the platinum activematerials of conventional alkaline fuel cell electrodes become veryinefficient at low temperatures. Unfortunately, conventional alkalinefuel cells still suffer from their own disadvantages.

For example, conventional alkaline fuel cells still use expensive noblemetal catalysts in both electrodes, which, as in the P.E.M. fuel cell,are susceptible to gaseous contaminant poisoning. The conventionalalkaline fuel cell is also susceptible to the formation of carbonatesfrom CO₂ produced by oxidation of the hydrogen electrode carbonsubstrates or introduced via impurities in the fuel and air used at theelectrodes. This carbonate formation clogs the electrolyte/electrodesurface and reduces/eliminates the activity thereof. The inventiondescribed herein eliminates this problem from the hydrogen electrode.

Fuel cells, like batteries, operate by utilizing electrochemicalreactions. Unlike a battery, in which chemical energy is stored withinthe cell, fuel cells generally are supplied with reactants from outsidethe cell. Barring failure of the electrodes, as long as the fuel,preferably hydrogen, and oxidant, typically air or oxygen, are suppliedand the reaction products are removed, the cell continues to operate.

Fuel cells offer a number of important advantages over internalcombustion engine or generator systems. These include relatively highefficiency, environmentally clean operation especially when utilizinghydrogen as a fuel, high reliability, few moving parts, and quietoperation. Fuel cells potentially are more efficient than otherconventional power sources based upon the Carnot cycle.

The major components of a typical fuel cell are the hydrogen electrodefor hydrogen oxidation and the oxygen electrode for oxygen reduction,both being positioned in a cell containing an electrolyte (such as analkaline electrolytic solution). Typically, the reactants, such ashydrogen and oxygen, are respectively fed through a porous hydrogenelectrode and oxygen electrode and brought into surface contact with theelectrolytic solution. The particular materials utilized for the oxygenelectrode and hydrogen electrode are important since they must act asefficient catalysts for the reactions taking place.

In an alkaline fuel cell, the reaction at the hydrogen electrode occursbetween the hydrogen fuel and hydroxyl ions (OH⁻) present in theelectrolyte, which react to form water and release electrons:

H₂+2OH⁻→2H₂O+2e ⁻(E₀=−0.828 v).

At the oxygen electrode, the oxygen, water, and electrons react in thepresence of the oxygen electrode catalyst to reduce the oxygen and formhydroxyl ions (OH^(−):)

O₂+2H₂O+4e ⁻→4OH⁻(E₀=−0.401 v).

The total reaction, therefore, is:

2H₂+O₂→2H₂O(E₀=−1.229 v)

The flow of electrons is utilized to provide electrical energy for aload externally connected to the hydrogen electrode and oxygenelectrode.

It should be noted that the hydrogen electrode catalyst of the alkalinefuel cell is required to do more than catalyze the reaction of H⁺ ionswith OH⁻ ions to produce water. Initially the hydrogen electrode mustcatalyze and accelerate the formation of H⁺ ions and e⁻ from H₂. Thisoccurs via formation of atomic hydrogen from molecular hydrogen. Theoverall reaction may be simplified and presented (where M is thecatalyst) as:

M+H₂→2M . . . H→M+2H⁺2e ⁻.

Thus the hydrogen electrode catalyst must not only efficiently catalyzethe electrochemical reaction for formation of water at the electrolyteinterface but must also efficiently dissociate molecular hydrogen intoatomic hydrogen. Using conventional anode material, the dissociatedhydrogen is transitional and the hydrogen atoms can easily recombine toform hydrogen if they are not used very efficiently in the oxidationreaction. With the hydrogen storage hydrogen electrode materials of theinventive instant startup fuel cells, hydrogen is stored in hydride formas soon as they are created, and then are used as needed to providepower.

In addition to being catalytically efficient on both interfaces, thecatalytic material must be resistant to corrosion by the alkalineelectrolyte. Without such corrosion resistance, the electrode wouldquickly succumb to the harsh environment and the cell would quickly loseefficiency and die.

One prior art fuel cell hydrogen electrode catalyst is platinum.Platinum, despite its good catalytic properties, is not very suitablefor wide scale commercial use as a catalyst for fuel cell hydrogenelectrodes, because of its very high cost resulting from the limitedworld supply. Also, noble metal catalysts like platinum, cannotwithstand contamination by impurities normally contained in the hydrogenfuel stream. These impurities can include carbon monoxide which may bepresent in hydrogen fuel or contaminants contained in the electrolytesuch as the impurities normally contained in untreated water includingcalcium, magnesium, iron, and copper.

The above contaminants can cause what is commonly referred to as a“poisoning” effect. Poisoning occurs where the catalytically activesites of the material become inactivated by poisonous species invariablycontained in the fuel cell. Once the catalytically active sites areinactivated, they are no longer available for acting as catalysts forefficient hydrogen oxidation reaction at the hydrogen electrode. Thecatalytic efficiency of the hydrogen electrode therefore is reducedsince the overall number of available catalytically active sites issignificantly lowered by poisoning. In addition, the decrease incatalytic activity results in increased over-voltage at the hydrogenelectrode and hence the cell is much less efficient adding significantlyto the operating costs. Overvoltage is the difference between the actualworking electrode potential under some conditions and it's equilibriumvalue, the physical meaning of overvoltage is the voltage required toovercome the resistance to the passage of current at the surface of thehydrogen electrode (charge transfer resistance). The overvoltagerepresents an undesirable energy loss which adds to the operating costsof the fuel cell.

In related work, U.S. Pat. No. 4,623,597 (“the '597 patent”) and othersin it's lineage, the disclosure of which is hereby incorporated byreference, one of the present inventors, Stanford R. Ovshinsky,described disordered multi-component multi-phase hydrogen storagematerials for use as negative electrodes in electrochemical cells forthe first time. In this patent, Ovshinsky describes how disorderedmaterials can be tailor made (i.e., atomically engineered) to greatlyincrease hydrogen storage and reversibility characteristics. Suchdisordered materials are amorphous, microcrystalline, intermediate rangeorder, and/or polycrystalline (lacking long range compositional order)wherein the polycrystalline material includes topological,compositional, translational, and positional modification and disorder.The framework of active materials of these disordered materials consistof a host matrix of one or more elements and modifiers incorporated intothis host matrix. The modifiers enhance the disorder of the resultingmaterials and thus create a greater number and spectrum of catalyticallyactive sites and hydrogen storage sites.

The disordered electrode materials of the '597 patent were formed fromlightweight, low cost elements by any number of techniques, whichassured formation of primarily non-equilibrium metastable phasesresulting in the high energy and power densities and low cost. Theresulting low cost, high energy density disordered material allowed thebatteries to be utilized most advantageously as secondary batteries, butalso as primary batteries.

Tailoring of the local structural and chemical order of the materials ofthe '597 patent was of great importance to achieve the desiredcharacteristics. The improved characteristics of the hydrogen electrodesof the '597 patent were accomplished by manipulating the local chemicalorder and hence the local structural order by the incorporation ofselected modifier elements into a host matrix to create a desireddisordered material. Disorder permits degrees of freedom, both of typeand of number, within a material, which are unavailable in conventionalmaterials. These degrees of freedom dramatically change a materialsphysical, structural, chemical and electronic environment. Thedisordered material of the '597 patent have desired electronicconfigurations which result in a large number of active sites. Thenature and number of storage sites were designed independently from thecatalytically active sites.

Multiorbital modifiers, for example transition elements, provided agreatly increased number of storage sites due to various bondingconfigurations available, thus resulting in an increase in energydensity. The technique of modification especially providesnon-equilibrium materials having varying degrees of disorder providedunique bonding configurations, orbital overlap and hence a spectrum ofbonding sites. Due to the different degrees of orbital overlap and thedisordered structure, an insignificant amount of structuralrearrangement occurs during charge/discharge cycles or rest periodsthere between resulting in long cycle and shelf life.

The improved battery of the '597 patent included electrode materialshaving tailor-made local chemical environments which were designed toyield high electrochemical charging and discharging efficiency and highelectrical charge output. The manipulation of the local chemicalenvironment of the materials was made possible by utilization of a hostmatrix which could, in accordance with the '597 patent, be chemicallymodified with other elements to create a greatly increased density ofelectro-catalytically active sites and hydrogen storage sites.

The disordered materials of the '597 patent were designed to haveunusual electronic configurations, which resulted from the varying3-dimensional interactions of constituent atoms and their variousorbitals. The disorder came from compositional, positional andtranslational relationships of atoms. Selected elements were utilized tofurther modify the disorder by their interaction with these orbitals soas to create the desired local chemical environments.

The internal topology that was generated by these configurations alsoallowed for selective diffusion of atoms and ions. The invention thatwas described in the '597 patent made these materials ideal for thespecified use since one could independently control the type and numberof catalytically active and storage sites. All of the aforementionedproperties made not only an important quantitative difference, butqualitatively changed the materials so that unique new materials ensued.

Disorder can be of an atomic nature in the form of compositional orconfigurational disorder provided throughout the bulk of the material orin numerous regions of the material. The disorder also can be introducedby creating microscopic phases within the material which mimic thecompositional or configurational disorder at the atomic level by virtueof the relationship of one phase to another. For example, disorderedmaterials can be created by introducing microscopic regions of adifferent kind or kinds of crystalline phases, or by introducing regionsof an amorphous phase or phases, regions of an amorphous phase or phasesin addition to regions of a crystalline phase or phases. The interfacesbetween these various phases can provide surfaces which are rich inlocal chemical environments which provide numerous desirable sites forelectrochemical hydrogen storage.

These same principles can be applied within a single structural phase.For example, compositional disorder is introduced into the materialwhich can radically alter the material in a planned manner to achieveimportant improved and unique results, using the Ovshinsky principles ofdisorder-on an atomic or microscopic scale.

Additionally, in copending U.S. application Ser. No. 09/524,116, ('116),the disclosure of which is hereby incorporated by reference, Ovshinskyhas employed the principles of atomic engineering to tailor materialswhich uniquely and dramatically advance the fuel cell art. The inventionof '116 application has met a need for materials which allow fuel cellsto startup instantaneously by providing an internal source of fuel, tooperate in a wide range of ambient temperatures to which a fuel cellwill be exposed to under ordinary consumer use and to allow the fuelcell to be run in reverse as an electrolyzer thereby utilizing/storingrecaptured energy. The hydrogen electrodes of the '116 fuel cells areformed from relatively inexpensive hydrogen storage materials which arehighly catalytic to the dissociation of molecular hydrogen and theformation of water from hydrogen and hydroxyl ions as well as beingcorrosion resistant to the electrolyte, resistant to contaminantpoisoning from the reactant stream and capable of working in a widetemperature range.

The next step in the evolution of the fuel cell would be to findsuitable materials to replace the expensive platinum oxygen electrodecatalysts of conventional fuel cells. It would also be advantageous toprovide the oxygen electrode with the ability to store chemical energy(possibly in the form of chemically bound oxygen) to assist in theinstant startup of the fuel cell as well as recapture energy Thus thereis a need within the art for such a material. The invention described inthis application is significant in that it provides the next step in thedevelopment of such electrochemical cells. With this invention, theoxygen electrode can be selected from a broad menu of available possibleredox couples. These redox couples in addition to providing a store ofchemical energy, allow the operating voltage of the fuel cell to beselected, by judicious choice of the redox couple used.

SUMMARY OF THE INVENTION

The object of the instant invention is a fuel cell which has the abilityto start up instantly and can accept recaptured energy such as that ofregenerative braking by operating in reverse as an electrolyzer. Theinstant startup fuel cells have increased efficiency and poweravailability (higher voltage and current) and a dramatic improvement inoperating temperature range (−20 to 150 ° C.) The fuel cells of theinstant invention also have additional degrees of freedom over the fuelcells of the prior art in that the voltage output of the cell can betailored and they are capable of storing regenerated energy.

The oxygen electrodes of the instant fuel cells operate through themechanism of redox reactions which uniquely provide multiple degrees offreedom in selecting the operating voltages available for such fuelcells. Such oxygen electrodes provide the fuel cells in which they areused, particularly alkaline fuel cells, with a level of chemical energystorage within the oxygen electrode itself. This means that such fuelcells will have a “buffer” or “charge” available within the oxygenelectrode at pre-startup.

The fuel cell oxygen electrode comprises an active material capable ofreversibly storing energy through the mechanism of a redox couple. Theelectrode has a first surface region situated to be exposed to molecularoxygen which includes a catalytically acting component promoting theabsorption of oxygen through said first surface region and into saidactive material to chemically charge said active material through oxygenabsorption.

The fuel cell oxygen electrodes of this invention may utilize redoxcouples, particularly metal/oxides couples selected from the group ofmetals consisting of copper, silver, zinc, cobalt and cadmium. Anotheruseful redox couple is the nickel hydroxide/nickel oxyhydroxide coupledisclosed herein above.

The fuel cell also employs an anode active material which has hydrogenstorage capacity. The anode active material is a hydrogen storage alloywhich has excellent catalytic activity for the formation of atomichydrogen from molecular hydrogen, outstanding catalytic activity towardthe formation of water from hydrogen ions and hydroxyl ions, and hasexceptional corrosion resistance toward the alkaline electrolyte of analkaline fuel cell. The anode active material is also low cost,containing no noble metals. The materials are robust and poisonresistant. The electrodes are easy to produce, by proven low costproduction techniques. The hydrogen electrode eliminates the use ofcarbon therein, thus helping to eliminating the carbonate poisoning ofthe fuel cell.

The anode active hydrogen storage alloys useful in the instant startupfuel cells reversibly absorbs and releases hydrogen and has a fasthydrogenation reaction rate and a long shelf-life. The hydrogen storagealloy is preferably selected from Alkaline Earth-Nickel alloys,Rare-Earth/Misch metal alloys, zirconium alloys, titanium alloys andmixtures or alloys thereof. The preferred hydrogen storage alloycontains enriched catalytic nickel regions distributed throughout theoxide surface of the particulate thereof. The catalytic nickel regionsare 50-70 Angstroms in diameter and vary in proximity from 2-300Angstroms (preferably from 50-100 Angstroms). An example of such analloy has the following composition:

(Base Alloy)_(a)Co_(b)Mn_(c)Fe_(d)Sn_(e)

where the Base Alloy comprises 0.1 to 60 atomic percent Ti, 0.1 to 40atomic percent Zr, 0 to 60 atomic percent V, 0.1 to 57 atomic percentNi, and 0 to 56 atomic percent Cr; b is 0 to 7.5 atomic percent; c is 13to 17 atomic percent; d is 0 to 3.5 atomic percent; e is 0 to 1.5 atomicpercent; and a+b+c+d+e=100 atomic percent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the reaction cycle of the cobalt hydroxide/oxyhydroxideredox couple.

FIG. 2 is a plot of electrode potential (volts) of the cathode versusthe current density (mA/cm²) for both the redox cathode of the instantinvention and the comparative cathode.

FIG. 3 is a stylized schematic depiction of a fuel cell oxygen electrodeused in the fuel cells of the instant invention.

FIG. 4 is a stylized schematic depiction of an inventive fuel cellhydrogen electrode used in the fuel cells of the instant invention.

FIG. 5 is a stylized schematic depiction of the instant startup alkalinefuel cell with hydrogen storage electrode and oxide couple electrode ina preferred embodiment of the instant invention.

FIG. 6 is a stylized schematic depiction of an energy supply systemincorporating the instant startup alkaline fuel cell of a preferredembodiment of the instant invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to catalysts for oxygen electrodes in fuel cellswhich operate through the mechanism of redox reactions. Such oxygenelectrodes, or oxidizable electrodes, provide the fuel cells in whichthey are used, particularly alkaline fuel cells, with a level ofelectrochemical energy storage within the oxygen electrode itself. Thismeans that such fuel cells will have a “buffer” or “charge” of reactantavailable within the oxygen electrode at pre-startup which, particularlycombined with hydrogen storage anodes described in copending U.S.application Ser. No. 09/524,116 (the disclosure of which is herebyincorporated by reference), yield instant start fuel cells in generaland more specifically to Ovonic instant start alkaline fuel cells. Suchfuel cells have a built in reserve of hydrogen within the hydrogenelectrode and oxygen electrode reactant (possibly oxygen) in the oxygenelectrode for instant startup (discussed herein below), and have theability to accept the energy of regenerative braking by acting as anelectrolyzer (also discussed herein below). The fuel cell has increasedefficiency and increased power capabilities as compared withconventional fuel cells of the prior art, while dramatically increasingthe operating temperature range of the cell (—20 to 150° C.) The fuelcell is easy to assemble and has the advantage of utilizing proven, lowcost production techniques.

The present invention also relates to fuel cell hydrogen electrodes andoxygen electrodes, and an energy supply system incorporating the presentfuel cell. The fuel cell hydrogen electrode includes materials whichhave inherent catalytic activity as well as hydrogen storage capacity.The oxygen electrode and hydrogen electrode materials do not need anynoble metals, and are therefore inherently low cost. The oxygenelectrode and hydrogen electrode materials are robust and long-lived,being resistant to poisoning. The hydrogen electrode does not utilizethe carbon substrates of the prior art. While a detailed discussion ofthe instant electrodes and their utilization in an alkaline fuel cell isdescribed herein below, it should be noted that the concepts of theinstant invention can be applied to other types of fuel cells (e.g.P.E.M. fuel cells).

In general, for such fuel cell oxygen electrodes, oxygen is generallyavailable to the oxygen electrode on a continuously supplied basis onone side thereof where the catalytically active material converts themolecular oxygen into atomic oxygen which is reduced at theelectrode/electrolyte to form hydroxyl ions. In prior art oxygenelectrodes, no storage of reactant occurs. That is oxygen travelsdirectly through the active materials and reacts at theelectrode/electrolyte interface. In the oxygen electrodes of the instantinvention, oxygen is stored in the oxygen electrode within thereversible redox couples, and is then available, at theelectrode/electrolyte interface of the oxygen electrode. Availableelectrons will then be generated through the electrochemical reactionwith the fuel. Thus the fuel cell will provide a constant supply ofelectricity at voltages characteristic of the redox couple, orelectrochemically reversible redox system (e.g. a metal and its oxide).Additionally, this added benefit may be obtained by redox couples otherthan between the simple combination of a metal and its oxidized form. Anexample of this is the redox couple of cobalt hydroxide/cobaltoxyhydroxide. With such a redox couple system, the fuel cell willprovide a potential whose theoretical voltage limit is the sum of thehydrogen electrode and oxygen electrode reactions. Certainly thetheoretical limit of voltage available is modified or limited by otherconsiderations, particularly including internal resistance of theelectrodes and the complete fuel cell system.

At the oxygen electrode, the oxygen, water, and electrons react in thepresence of the cathode active material to reduce the oxygen and formhydroxyl ions (OH⁻):

O₂+2H₂O+4e ⁻→4OH⁻.

The flow of electrons is utilized to provide electrical energy for aload externally connected to the hydrogen electrode and oxygenelectrode. That load is available to be filled by any number of needsincluding, but not limited to, powering motive vehicles, lightingdevices, heating or cooling devices, power tools, entertainment devices,and other electricity-consuming devices too numerous to mention.

Numerous redox couples exist and may be used to form the cathode of thisinvention. When such couples are used, cycling transition from theoxidized form to the reduced form is accomplished repeatedly andcontinuously. From a practical point of view, the ability to withstandsuch cycling is preferred. While not wishing to be bound by theory, theinventors believe that the equations representing some of the manyavailable reactions for the oxygen side of the fuel cell are presentedbelow.

Using a nickel hydroxide/oxyhydroxide redox couple, the following isbelieved to be the useful fuel cell valency change mechanisms:

2NiOOH+2e ⁻+2H₂O→2Ni(OH)₂+2OH⁻(Electrochemical reduction)

C+O₂→C . . . O (Intermediate step for adsorbed oxygen)

C . . . O+2Ni(OH)₂→2NiOOH+H₂O+C (chemical oxidation)

or during regenerative braking or other charging input:

Ni(OH)₂+(OH)⁻→NiO(OH)+H₂O+e ⁻

Using a copper/copper oxide couple, the following is believed to be theuseful fuel cell valency change mechanisms:

{4C+4H₂O+4e ⁻→4C . . . H+4(OH)⁻(Electrochemical Water discharge)

{4C . . . H+2CuO→2Cu+2H₂O+4C

2Cu+O2→2CuO (Chemical Oxidation)

O₂+2H₂O+4e ⁻→4(OH)⁻(Overall)

or during regenerative breaking or other charging input:

Cu+2(OH⁻)→CuO+H₂O+e ⁻

Using a silver/silver oxide couple, the following is believed to be theuseful fuel cell oxygen-side reactions:

4Ag+O2→2Ag₂O (Chemical Oxidation)

2Ag₂O+H₂O+4e ⁻→4Ag+4(OH)⁻(Electrochemical reduction)

O₂+2H₂O+4e ⁻4(OH)⁻(Overall)

or during regenerative breaking or other charging input:

2Ag+2(OH)⁻→Ag₂O+H₂O+2e ⁻

In all the above reactions, the overall reaction is still oxygenreduction. In these reactions the pathway chosen is not a directelectrochemical reduction of O₂, but via a redox reaction. This givesanother degree of freedom to choose the right redox reaction, matchingthe kinetics and the operating potential desired.

Although the previously mentioned redox couples may be used in theoxygen electrodes, the oxygen electrodes of the present inventionpreferably utilizes a cobalt hydroxide/oxyhydroxide redox couple. Thecobalt hydroxide/oxyhydroxide redox couples are preferred due to thereduction and oxidation reaction rates being almost identical. In thiscase, the rate of oxygen being produced is nearly the same as the oxygenbeing consumed thereby avoiding a limiting current density. This allowsthe electrode to discharge at a high rate by reducing polarization inthe cathodes.

As shown in FIG. 1, when using the cobalt hydroxide/oxyhydroxide redoxcouple, either cobalt oxide (CoO), cobalt hydroxide (2Co(OH)₂), orcobalt oxyhydroxide 2CoO(OH) can be used to start the process. Cobaltoxide, cobalt hydroxide, and cobalt oxyhydroxide are all commerciallyavailable and can be easily obtained. Where cobalt oxide is used, cobalthydroxide is formed through the following reactions:

CoO+2OH⁻→CoO₂ ⁻²+H₂O

CoO₂ ⁻²+2H₂O→Co(OH)₂+2OH⁻

Overall: CoO+H₂O→Co(OH)₂

The cobalt hydroxide undergoes an oxidation reaction to produce cobaltoxyhydroxide. The cobalt oxyhydroxide undergoes a reduction reaction toproduce cobalt hydroxide and 2 electrons. The valency change mechanismsfor the cobalt hydroxide/oxyhydroxide redox couple are as follows:

2Co(OH)₂+O₂→2CoO(OH)+H₂O₂ (Chemical Oxidation)

2CoO(OH)+2H₂O+2e ⁻→2Co(OH)₂+2(OH)⁻(Electrochemical red.)

O₂+2H₂O+2e ⁻→2(OH)⁻+H₂O₂ (Overall)

The performance of the cobalt hydroxide/oxyhydroxide (▪) redox couple isshown in FIG. 2. Also shown is a conventional silver catalyst (♦) and asilver catalyst modified with an active material component (▾). Thecobalt hydroxide/oxyhydroxide redox couple performs considerably betterthan the conventional silver catalyst and the modified silver catalystin oxygen electrodes at room temperature.

The fuel cell oxygen electrodes of the instant invention also include acatalytic material which promotes the dissociation of molecular oxygeninto atomic oxygen (which reacts with the redox couple). A particularlyuseful catalyst is carbon. As discussed herein below this carbon shouldbe very porous and may be electrically conductive.

The oxygen electrodes may contain an active material component which iscatalytic to the dissociation of molecular oxygen into atomic oxygen,catalytic to the formation of hydroxyl ions (OH⁻) from water and oxygenions, corrosion resistant to the electrolyte, and resistant topoisoning. A material useful as an active material in the oxygenelectrode is on a host matrix including at least one transition metalelement which is structurally modified by the incorporation of at leastone modifier element to enhance its catalytic properties. Such materialsare disclosed in U.S. Pat. No. 4,430,391 ('391) to Ovshinsky, et al.,published Feb. 7, 1984, the disclosure of which is hereby incorporatedby reference. Such a catalytic body is based on a disorderednon-equilibrium material designed to have a high density ofcatalytically active sites, resistance to poisoning and long operatinglife. Modifier elements, such as La, Al, K, Cs, Na, Li, Ga, C, and Ostructurally modify the local chemical environments of the host matrixincluding one or more transition elements such as Mn, Co and Ni to formthe catalytic materials of the oxygen electrode. These low over-voltage,catalytic materials increase operating efficiencies of the fuel cells inwhich they are employed.

The oxygen electrode may be formed the same as conventional oxygenelectrodes which use platinum catalysts, but the non-noble-metalcatalysts described above are substituted for the platinum. Thenon-noble catalysts are finely divided and disbursed throughout a porouscarbon matte-like material. The material may or may not have aconductive substrate as needed.

The fuel cell oxygen electrodes of this invention utilize redox couples,particularly a cobalt hydroxide/oxyhydroxide couple. The oxygenelectrodes of the instant invention may also include a catalyticmaterial which promotes and speeds the dissociation of molecular oxygeninto atomic oxygen (which reacts with the redox couple). A particularlyuseful catalyst is carbon. This carbon should be very porous and may beelectrically conductive.

The oxygen electrode also needs a barrier means to isolate theelectrolyte, or wet, side of the oxygen electrode from the gaseous, ordry, side of the oxygen electrode. A beneficial means of accomplishingthis is by inclusion of a hydrophobic component comprising a halogenatedorganic compound, particularly polytetrafluoroethylene (PTFE) within theelectrode.

The oxygen electrodes, may also include a current collector or currentcollecting system extending within said active material. The currentcollector may comprise an electrically conductive mesh, grid, foam orexpanded metal. The choice of such collection systems may be madeaccording to electrode manufacturing or production system needs.

The oxygen electrode in the preferred embodiment of the presentinvention has a layered structure and is shown in FIG. 3. The layeredstructure promotes oxygen dissociation and absorption within the oxygenelectrode 10. Each oxygen electrode 10 is composed of a thin A layer 11,a B layer 12, and a C 13 layer with a current collector grid 14 embeddedwithin the C layer 13. The C layer 13 is on the electrolyte contactingside 15 of the oxygen electrode 10 and the A layer 11 is on the oxygencontacting side 16 of the oxygen electrode 10. The A layer 11 may becomposed of carbon particles coated with polytetrafluoroethylene (PTFE).The carbon particles may be acetylene black, which is well known in theart. The carbon/PTFE mixture may contain approximately 30% PTFE with theremainder comprising acetylene black carbon, thereby making the carbon30% teflonated. The 30% teflonated acetylene black is mixed with thecobalt oxide catalyst. The cobalt oxide comprises approximately 10% ofthe A layer while the 30% teflonated acetylene black comprises theremaining 90% of the A layer. The B layer 12 may be wholly composed ofcarbon particles coated with polytetrafluoroethylene. The carbonparticles may be carbon black known as Vulcan XC-72 carbon (Trademark ofCabot Corp.), which is well known in the art. The B layer 12 may containapproximately 40 percent by weight polytetrafluoroethylene with theremainder consisting of carbon particles. The C layer 13 may containapproximately 65 percent a teflonated mixture of two different types ofcarbon, 15% graphite, and 20% cobalt oxide. The teflonated carbonmixture comprises Acetylene Black and Cabot Black Pearl 2000 mixed in anapproximate 3:1 ratio and teflonated to 20%. The carbon added to the Blayer 12 is carbon black known as Black Pearls 2000 (Trademark of CabotCorp.). The graphite is preferably TIMREX SFG 44 graphite (Trademark ofTimcal Group). Embedded throughout the C layer is a current collectorgrid serving both as a substrate and a current collector. Examples ofcurrent collector grids include, but are not limited to, mesh, grid,matte, expanded metal, foil, foam and plate. The cobalt oxide may alsocontain a lithium-aluminum alloy, gallium, or other modifiers forimproved performance.

Reactive elements such as lithium may be added to the redox couple inthe form of a non-reactive alloy such as a LiAl alloy. That is, lithiumalone as an individual element is extremely reactive with oxygen andwater vapor, therefore it is advisable to incorporate the element intothe redox couple in the form of an alloy with aluminum which is notreactive in this way. Other elements which may be alloyed with thelithium include boron and silicon. Specifically the LiAl alloy is a50:50 At. % alloy.

In a fuel cell the oxygen electrode just described is used inconjunction with a hydrogen electrodes. While any functional hydrogenelectrode may be used with the inventive oxygen electrode, preferredembodiments of the fuel cells of this invention will include hydrogenelectrodes employing hydrogen storage alloy active materials. It shouldbe noted that the preferred hydrogen electrode catalyst of the alkalinefuel cell is required to do more than catalyze the reaction of H⁺ ionswith OH⁻ ions to produce water. Initially the hydrogen electrode mustcatalyze and accelerate the formation of H⁺ ions. This occurs viaformation of atomic hydrogen from molecular hydrogen. The overallreaction can be seen as (where M is the hydrogen storage anode activealloy material):

M+H₂→M . . . H→MH→M+H⁺ +e ⁻.

That is, molecular hydrogen (H₂) is converted to adsorbed atomichydrogen (M . . . H) onto the surface of the hydrogen electrode. Thisadsorbed hydrogen is very quickly converted to a metal hydride (MH) inthe bulk of the hydrogen storage alloy. This hydride material is thenconverted to ionic H⁺ releasing an electron e⁻. The ionic hydrogenreacts with a hydroxyl ion in the electrolyte to produce water and theelectron is released into the external load circuit. Thus the hydrogenelectrode catalyst must not only efficiently catalyze the formation ofwater at the electrolyte interface but must also efficiently dissociatemolecular hydrogen into ionic hydrogen. Using conventional hydrogenelectrode material, the dissociated hydrogen is transitional and thehydrogen atoms can easily recombine to form hydrogen if they are notused very quickly in the oxidation reaction. With hydrogen storageelectrode materials, hydrogen is trapped in hydride form as soon ashydrides are created. The hydrogen, as electrochemically released intothe electrolyte, are then used as needed to provide the fuel cell'selectrical power output.

In addition to being catalytically efficient on both interfaces, thehydrogen electrode catalytic material must be resistant to corrosion bythe alkaline electrolyte. Without such corrosion resistance, theelectrode would quickly succumb to the harsh environment and the cellwould quickly lose efficiency and die.

One prior art fuel cell hydrogen electrode catalyst is platinum.Platinum, despite its good catalytic properties, is not very suitablefor wide scale commercial use as a catalyst for fuel cell hydrogenelectrodes, because of its very high cost. Also, noble metal catalystslike platinum, also cannot withstand contamination by impuritiesnormally contained in the hydrogen fuel stream. These impurities caninclude carbon monoxide (which may be present in hydrogen fuel) orcontaminants contained in the electrolyte such as the impuritiesnormally contained in untreated water including calcium, magnesium,iron, and copper.

The above contaminants can cause what is commonly referred to as a“poisoning” effect. Poisoning occurs where the catalytically activesites of the material become inactivated by poisonous species invariablycontained in the fuel cell. Once the catalytically active sites areinactivated, they are no longer available for acting as catalysts forefficient hydrogen oxidation reaction at the hydrogen electrode. Thecatalytic efficiency of the hydrogen electrode therefore is reducedsince the overall number of available catalytically active sites issignificantly lowered by poisoning. In addition, the decrease incatalytic activity results in increased overvoltage at the hydrogenelectrode and hence the cell is much less efficient adding significantlyto the operating costs. Overvoltage is the difference between theelectrode potential and it's equilibrium value, the physical meaning ofover-voltage is the voltage required to overcome the resistance to thepassage of current at the surface of the hydrogen electrode (chargetransfer resistance). The overvoltage represents an undesirable energyloss which adds to the operating costs of the fuel cell.

The hydrogen electrodes may be generally composed of an anode activematerial having hydrogen storage capacity. The anode active material isdesigned to have a high density of active catalytic sites, resistance topoisoning; and long operating life to provide efficient low cost fuelcell operation.

An anode active material of the instant invention may be a composite ofa hydrogen storage material and an additional catalytic material. Thepreferable anode active material is one which can reversibly absorb andrelease hydrogen irrespective of the hydrogen storage capacity and hasthe properties of a fast hydrogenation reaction rate, a good stabilityin the electrolyte and a long shelf-life. It should be noted that, byhydrogen storage capacity, it is meant that the material stores hydrogenin a stable form, in some nonzero amount higher than trace amounts.Preferred materials will store about 0.1 weight % hydrogen or more.Preferably, the alloys include, for example, rare-earth/Misch metalalloys, zirconium and/or titanium alloys or mixtures thereof. The anodematerial may even be layered such that the material on the hydrogencontacting surface is formed from a material which has been specificallydesigned to be highly catalytic to the dissociation of molecularhydrogen into atomic hydrogen, while the material on electrolytecontacting surface is designed to be highly catalytic to the formationof water from hydrogen and hydroxyl ions.

Certain hydrogen storage materials are exceptionally useful as alkalinefuel cell hydrogen electrode materials. The useful hydrogen storagealloys have excellent catalytic activity for the formation of hydrogenions from molecular hydrogen and also have superior catalytic activitytoward the formation of water from hydrogen ions and hydroxyl ions. Inaddition to having exceptional catalytic capabilities, the materialsalso have outstanding corrosion resistance toward the alkalineelectrolyte of the fuel cell. In use, the alloy materials act as 1) amolecular hydrogen decomposition catalyst throughout the bulk of thehydrogen electrode; and 2) as an internal hydrogen storage buffer toinsure that a ready supply of hydrogen atoms is always available at theelectrolyte contacting surface.

Specific alloys useful as the anode material are alloys that containenriched catalytic nickel regions of 50-70 Angstroms in diameterdistributed throughout the oxide interface which vary in proximity from2-300 Angstroms preferably 50-100 Angstroms, from region to region. As aresult of these nickel regions, the materials exhibit significantcatalysis and conductivity. The density of Ni regions in the alloysprovide powder particles having an enriched Ni surface. The mostpreferred alloys having enriched Ni regions are alloys having thefollowing composition:

(Base Alloy)_(a)Co_(b)Mn_(c)Fe_(d)Sn_(e)

where the Base Alloy comprises 0.1 to 60 atomic percent Ti, 0.1 to 40atomic percent Zr, 0 to 60 atomic percent V, 0.1 to 57 atomic percentNi, and 0 to 56 atomic percent Cr; b is 0 to 7.5 atomic percent; c is 13to 17 atomic percent; d is 0 to 3.5 atomic percent; e is 0 to 1.5 atomicpercent; and a+b+c+d+e=100 atomic percent. Such materials are disclosedin U.S. Pat. No. 5,536,591 to Fetcenko et al., published Jul. 16, 1996,the disclosure of which is hereby incorporated by reference.

The hydrogen electrodes in the preferred embodiment of the presentinvention have a layered structure and are exemplified in FIG. 4. Thelayered structure promotes hydrogen dissociation and absorption withinthe hydrogen electrode 20. Each hydrogen electrode 20 is composed of anactive material layer 21, a current collector grid 22, and a porouspolytetrafluoroethylene layer 23. The active material layer 21 isdisposed between the current collector grid 22 and thepolytetrafluoroethylene layer 23. The active material layer 21 may bedispersed throughout the current collector grid 22. Examples of currentcollector grids include, but are not limited to, mesh, grid, matte,expanded metal, foil, foam and plate. The current collector grid may becomposed of a conductive material such as nickel. Other conductivematerials may be substituted as required by design constraints. Thepolytetrafluoroethylene layer 23 may be approximately 0.0007 inchesthick. The current collector grid 22 is on the electrolyte contactingside 24 of the hydrogen electrode 20 and the polytetrafluoroethylenelayer 23 is on the hydrogen contacting side 25 of the hydrogen electrode20.

The active material layer 21 may be composed of Misch metal nickelalloy, Raney nickel, graphite, and polytetrafluoroethylene powder. Apreferred composition of the active material layer 21 is by weight 35%Mischmetal nickel alloy, 46% Raney nickel, 4% graphite, and 15%polytetrafluoroethylene. The most preferred Misch metal nickel alloy hasthe following composition by weight percent: 50.07% Ni, 10.62% Co, 4.6%Mn, 1.8% Al, 20.92% La, 8.63% Ce, 0.87% Pr, and 2.49% Nd. The graphitemay be one with isotropic shape having high electrical and thermalconductivity. A typical example of such graphite is called TIMREX KS-75(Trademark of Timcal Group). Raney nickel and polytetrafluoroethyleneare well known in the art and do not need any further discussion.

It should be noted that the hydrogen electrode and oxygen electrodeactive materials of the instant invention are robust and very resistantto poisoning. This is true because the increased number of catalyticallyactive sites of these materials not only increases catalytic activity,but enables the materials to be more resistant to poisoning, becausewith materials of the present invention numerous catalytically activesites can be sacrificed to the effects of poisonous species while alarge number of non-poisoned sites still remain active to provide thedesired catalysis. Also, some of the poisons are inactivated by beingbonded to other sites without effecting the active sites.

FIG. 5 is a stylized schematic depiction of an alkaline fuel cell 30incorporating the oxygen electrode 10 and the hydrogen electrode 20 ofthe instant invention. The fuel cell 30 consists of three generalsections: 1) a hydrogen electrode section, which includes the hydrogenelectrode 20 and a hydrogen supply compartment 31; 2) the electrolytecompartment 32; and 3) the oxygen electrode section, which includes theoxygen electrode 10 and the oxygen (air) supply compartment 32.

In the hydrogen electrode section, hydrogen or hydrogen containing gasmixtures is supplied under pressure to the hydrogen supply compartment31 through hydrogen inlet 33. Hydrogen is then absorbed through thehydrogen contacting surface 25 into the hydrogen electrode 20. Theabsorbed hydrogen is catalytically broken down by the anode activematerial into atomic hydrogen which is stored in the hydrogen storagematerial as a hydride, and then finally reacts at the electrolytecontacting surface 24 with hydroxyl ions to form water. It should benoted that the heat of hydride formation helps to warm the fuel cell toit's optimal operating temperature. Any unabsorbed hydrogen and othercontaminant gases or water vapor in the hydrogen supply are ventedthrough outlet 34. The gases that are vented may be recycled if enoughhydrogen is present to warrant recovery. Otherwise the hydrogen may beused to provide a source of thermal energy if needed for othercomponents such as a hydride bed hydrogen storage tank.

The electrolyte compartment 32 holds (in this specific example) anaqueous alkaline electrolyte in intimate contact with the hydrogenelectrode 20 and the oxygen electrode 10. The alkaline solution is wellknown in the art and is typically a potassium hydroxide solution. Theelectrolyte provides hydroxyl ions which react with hydrogen ions atelectrolyte contacting surface 24 of the hydrogen electrode 20 and watermolecules which react with oxygen ions at the electrolyte contactingsurface 15 of the oxygen electrode 10. The electrolyte is circulatedthrough compartment 32 via inlet 35 and outlet 36 (in alternativeembodiments, the electrolyte may be deliberately immobilized as byjelling, etc.) The circulated electrolyte may be externally heated orcooled as necessary, and the concentration of the electrolyte can beadjusted (as via wicking, etc.) as needed to compensate for the waterproduced by the cell and any losses due to evaporation of water throughthe electrodes. Systems for conditioning the fuel cell electrolyte arewell known in the art and need not be further described in detailherein.

In the oxygen electrode section, oxygen, air, or some other oxygencontaining gaseous mixture is supplied to the oxygen supply compartment32 through oxygen inlet 37. Oxygen is then absorbed through the oxygencontacting surface 16 into the oxygen electrode 10. The absorbed oxygenis catalytically broken down by the cathode active material into ionicoxygen, which finally reacts at the electrolyte contacting surface 15(via the redox couple) with water molecules to form hydroxyl ions. Anyunabsorbed oxygen and other gases in the feed (e.g. nitrogen, carbondioxide, etc.) or water vapor in the oxygen supply are vented throughoutlet 38.

Such fuel cells may, as a system, further comprise an electrolyteconditioning means for conditioning the electrolyte. This electrolyteconditioning system will not only adjust the temperature of theelectrolyte (for optimal fuel cell performance) but will also removewater from the electrolyte. The water removal is necessary because wateris produced as a by-product of the fuel cell's electrochemical hydrogenoxidation reaction. This water, if not removed would dilute theelectrolyte, thus impeding the optimal performance of the fuel cell.

These fuel cells will further include, as a system, a hydrogen supplysource including means for continuously supplying fuel, particularlymolecular hydrogen, to the hydrogen electrode's first surface region; anoxygen supply source which includes means for continuously supplyingmolecular oxygen to the oxygen electrode's first surface region; and anelectrolyte conditioning system which includes means for continuouslyconditioning the electrolyte, thereby enabling continuous operation ofthe fuel cell as an electrical power source.

Fuel cells of the instant invention using oxygen electrode with redoxcouples, particularly in combination with the hydrogen storage anodes ofthe '116 application provide the ability to recapture reverse electricalpower flow from an external circuit into said fuel cell,electrolytically producing hydrogen and oxygen which are absorbed andstored through the mechanism of the redox couple in the oxygen electrodeand the hydrogen storage material in the hydrogen electrode. When theinstant fuel cell is run in reverse, as an electrolyzer, during anenergy recapture process such as regenerative braking, water iselectrolyzed into hydrogen and oxygen. In this regenerative brakingmode, the electric motors reverse their roles and become generatorsusing up the kinetic energy of the motion. This causes a spike ofcurrent which amounts to about 10% of the normal operating load. Aconventional fuel cell (alkaline or P.E.M.) cannot accept such surges.This feedback of energy would cause rapid hydrogen and oxygen evolutionwhich would cause the catalysts to lose their integrity and adhesionthereby undermining the overall system performance.

In the inventive fuel cell, this will not be a problem, because thehydrogen storage anode and the oxide couple oxygen electrode will takethe surge current and become charged with the produced hydrogen oroxygen respectively.

FIG. 6 is a stylized schematic depiction of an energy supply system 40incorporating the alkaline fuel cell 30 of the instant invention. Theenergy supply system also includes a source of hydrogen 41. The sourcemay be of any known type, such as a hydride bed storage system, acompressed hydrogen storage tank, a liquid hydrogen storage tank, or ahydrocarbon fuel reformer. The preferred source is a metal hydridestorage system. The hydrogen from the source 41 is transported to thefuel cell 30 via input line 50, and excess gases are vented throughoutput line 51. A portion of the gases from output line 51 may berecycled to input line 50 through recycle line 52. The energy supplysystem also includes a source of oxygen, which is preferably air foreconomic reasons. The air is drawn into line 53 and then can be passedthrough a carbon dioxide scrubber 42. The air is then transported to thefuel cell 30 via input line 54. Excess air and unused gases are ventedthrough output line 55. Since this gas stream contains no harmful gases,it may be vented to the environment directly.

The energy supply system may also include an electrolyte recirculationsystem. The electrolyte from the fuel cell 30 is removed through outputline 56 and sent to an electrolyte conditioner 43. The electrolyteconditioner 43 heats or cools the electrolyte as needed and removes/addswater as necessary. The conditioned electrolyte is then returned to thefuel cell 30 via input line 57.

Finally the energy supply system includes electrical leads 44 and 45which supply electricity from the fuel cell 30 to a load 46. The loadcan be any device requiring power, but particularly contemplated is thepower and drive systems of an automobile.

The instant fuel cell and energy supply systems incorporating it areparticularly useful for applications in which instant start and energyrecapture are requirements thereof, such as for example in powering avehicle. For instance, in consumer vehicle use, a fuel cell that has thebuilt in fuel and oxidizer storage of the instant invention has theadvantage of being able to start producing energy instantly from thereactants stored in it's electrodes. Thus, there is no lag time whilewaiting for hydrogen to be supplied from external sources. Additionally,because hydrogen and oxygen can be adsorbed and stored in the respectiveelectrode materials of the fuel cell, energy recapture can be achievedas well. Therefore, activities such as regenerative braking, etc., canbe performed without the need for an battery external to the fuel cellbecause any reactants produced by running the fuel cell in reverse willbe stored in the electrodes of the fuel cell. Therefore, in essence,fuel cells employing the instant active electrode materials are theequivalent of a fuel cell combined with a battery. In such a systememploying the redox couples oxygen is also able to be stored within theelectrode to a significant degree as an oxidized component of thecouple, preferably a metal/metal oxide couple, a hydroxide/oxyhydroxide,or combinations thereof.

The novel electrochemical cell of the present invention also enables thepractice of the method of the invention which, in one embodimentthereof, comprises the indirect and continuous introduction of both thefuel, preferably hydrogen, and the reactant which oxidizes the fuel,preferably oxygen, for the continuous operation of the electrochemicalcell as a fuel cell. That is, the hydrogen is, during operation,continuously introduced through a catalytic region in the negativeelectrode and continuously stored as a hydride in a region of materialin the negative electrode which is capable of reversibly storing andreleasing hydrogen. Simultaneously therewith hydrogen iselectrochemically released from the negative electrode, on itselectrolyte side, to participate in the cell reaction process so thatcontinuous supply at the gas side, storage within, and release ofhydrogen at the electrolyte side of, the negative electrode is madepossible.

At the same time oxygen is continuously introduced at the gas side ofthe positive electrode through a catalytic region and chemically storedas a material in the form of the charged state of an oxide couple whichparticipates in the cell reaction. Simultaneously with the introductionand chemical storage of the oxygen as just explained the material of theoxide couple which is in the charged state participates in the cellreaction to generate electrical power. Thus an electrochemical cell iscontinuously operated through the supply to the gas side, storagewithin, and release from the electrolyte side of, the oxidant so thatoperation as a fuel cell is enabled. The unique method of the inventionof operation of an electrochemical cell as a fuel cell is thus madepossible. In the situations in which the fuel cell is run “backwards” oras an electrolyzer to recapture and store energy, such as for example,during regenerative braking, the operating nature as described earlierwould not be considered to be disruptive to “continuous” operation.

The drawings, discussion, descriptions, and examples of thisspecification are merely illustrative of particular embodiments of theinvention and are not meant as limitations upon its practice. It is theappended claims, including all equivalents, that define the scope of theinvention.

What is claimed is:
 1. In a fuel cell, an oxygen electrode including acathode active material having oxygen storage capacity comprising: aredox couple which provides for said oxygen storage capacity.
 2. Thefuel cell oxygen electrode of claim 1, further including a hydrophobiccomponent.
 3. The fuel cell oxygen electrode of claim 2, wherein saidhydrophobic component comprises polytetrafluoroethylene (PTFE).
 4. Thefuel cell oxygen electrode of claim 3, wherein said PTFE is at least oneof: a) intimately mixed with said cathode active material; b) gradedwithin said cathode active material; or c) a separate layer within saidoxygen electrode.
 5. The fuel cell oxygen electrode of claim 1, furtherincluding a current collector extending within said active material. 6.The fuel cell oxygen electrode of claim 5 wherein said current collectorcomprises an electrically conductive mesh, grid, foam or expanded metal.7. The fuel cell oxygen electrode of claim 1, further including acatalytic carbon component.
 8. In a fuel cell, said fuel cell includinga cathode active material having oxygen storage capacity comprising: aredox couple which provides for said oxygen storage capacity.
 9. Thefuel cell of claim 8, wherein said oxygen storage capacity provides saidfuel cell with instant startup capability.
 10. The fuel cell of claim 8,wherein said oxygen storage capacity provides said fuel cell with theability to accept recaptured energy by running in reverse as anelectrolyzer.
 11. The fuel cell of claim 8, wherein said oxygenelectrode further includes a hydrophobic component which comprisespolytetrafluoroethylene.
 12. The fuel cell of claim 8, wherein saidoxygen electrode further includes a current collector extending withinsaid active material.
 13. The fuel cell of claim 12, wherein saidcurrent collector comprises an electrically conductive mesh, grid, foamor expanded metal.
 14. The fuel cell of claim 8, wherein said oxygenelectrode further includes a catalytic carbon component.
 15. The fuelcell of claim 8, wherein said fuel cell further includes a hydrogenelectrode, said hydrogen electrode including an anode active materialhaving hydrogen storage capacity.
 16. The fuel cell of claim 15, whereinsaid hydrogen storage capacity additionally provides said fuel cell withinstant startup capability.
 17. The fuel cell of claim 15, wherein saidhydrogen storage capacity additionally provides said fuel cell with theability to accept recaptured energy by running in reverse as anelectrolyzer.
 18. The fuel cell of claim 15, wherein said hydrogenstorage capacity provides thermal energy to said fuel cell via the heatof formation of the hydride thereof.
 19. The fuel cell of claim 15,wherein said anode active material comprises a hydrogen storage alloy.20. The full cell of claim 19, wherein said hydrogen storage alloy isselected from the group consisting of Alkaline Earth-Nickel alloys, RareEarth/Misch metal alloys, zirconium alloys, titanium alloys, andmixtures or alloys thereof.